Preparation of Lithium Sulfur Battery Materials and Modification by Physical Vapor Deposition

Author:Zhang Jing

Supervisor:li he qin


Degree Year:2019





The rapid development of electronic communication equipment and electric vehicles requires new batteries with high specific energy,high safety,low cost,environmental friendliness and durability.It is difficult for commercial lithium-ion battery to further improve its endurance ascribing to the specific capacity of cathode materials and safety.Lithium-sulfur secondary batteries have become a research hotspot in the field of new energy owing to the high theoretical specific capacity of1675mAh/g,abundant sources,low cost and environmental friendliness.However,there are disadvantages of the system,including poor conductivity,low utilization of active materials,shuttle effect and so on,which hinder its commercialization.In order to solve the problems,the modification of lithium-sulfur batteries was studied in this thesis from the aspects of cathode materials,separator and“sandwich”electrode,and the mechanism of high performance was analyzed.The main innovations are as follows:mesoporous carbon is prepared by hard-template method;the influences of template dosage on the properties of mesoporous carbon and Li-S batteries are studied;the cathode and seperator are modified by TiO2 nanotubes and nano TiO2 particles to improve cycle performance and Coulomb efficiency;Al and Ti particles are deposited on the cathode and carbon film by physical vapor deposition,which improves the conductivity of the cathode and new Li-S batteries with sandwich structure are developed.In order to enhance the utilization of active materials,three kinds of mesoporous carbon were prepared by hard-template method with polyvinyl alcohol as precursor carbon source and nano-CaCO3 as the template to synthesis S/MC cathode materials.The influences of the ratios of nano-CaCO3 template on the pore-forming mechanism of mesoporous carbon and the electrochemical performance of Li-S battery were investigated.The results show that the mesoporous carbon MC2,prepared with the mass ratio of CaCO3:PVA=1:1.5,has the specific surface area of850m2/g and pore volume of 0.57cm3/g.The Li-S battery S/MC2 deliverd an initial discharge specific capacities of 1384mAh/g and 1258mAh/g at 0.5C and 1C rates respectively.After 100 cycles,the reversible specific capacities of 882mAh/g and861mAh/g were maintained severally,showing high capacity retention and cyclic reversibility.The modified lithium-sulfur batteries S/AC/-NT/Tnt and S/AC/(Cel-NT/Tnt)were prepared with the mixture of nano-TiO2 and TiO2 nanotubes,prepared by anodic oxidation,as the additive for cathode materials and the coating for Celgard 2400separator.The initial discharge specific capacities of S/AC/(Cel-NT/Tnt)at 0.5C and1C were 1215mAh/g and 1035mAh/g,respectively,and the capacity of 899mAh/g and 873mAh/g were obtained after 100 cycles.Furthermore,the Coulomb efficiency remained above 98%after 200 cycles,representing good persistence and cycle stability.Physical vapor deposition was employed to modify the cathode material and carbon interlayer in order to enhance the electrochemical performance of Li-S batteries.Cathode material S/AC was synthesized with commercial activated carbon as the conductive matrix.Al and Ti were deposited on the surface of S/AC electrode by radio frequency magnetron sputtering to improve the conductivity.Besides,Al and Ti particles were embedded into the carbon pores on the surface of the electrode,which restrained the shuttle effect to some extent and improved the cycle stability of the batteries.The cathode coated with Al delivered an initial discharge specific capacity of 1257mAh/g and 977mAh/g at the 100th cycle at 0.5C rate,with the coulomb efficiency greater than 97%.The "sandwich" batteries were assembled with the cathode,conductive carbon film and the separator.Al and Ti were deposited on the filter paper surface by radio frequency magnetron sputtering,and the composite carbon films were prepared by high-temperature carbonization of the coated filter paper and emloyed as conductive interlayer between the cathode and separator.Al and Ti have excellent electrical conductivity and ductility,which enables good conductivity and flexibility of carbon film and helps to increase the kinetic reaction rate as well as buffer the volume swell of cathode.The initial discharge specific capacities of 1394mAh/g and1273mAh/g were obtained at 0.5C and 1C respectively for the battery containing a Al-coated carbon interlayer.The capacity of 889 mAh/g and 924mAh/g were achieved after 100 cycles,respectively.The coulomb efficiency after 200 cycles was still higher than 98%,meaning that the shuttle effect was alleviated efficiently and the utilization sulfur has been increased.Magnetron sputtering has been proved to be an effective method to promote the electrochemical property of Li-S batteries.